Field of the Invention
The present invention relates to a rotary tool, and more particularly to a drilling tool, which extends along a longitudinal axis and has a front end surface with a brad point, with a first major cutting edge extending outward up to an outer edge corner; and with a first free surface segment adjoining the major cutting edge; and with a second free surface segment adjoining said first free surface segment. The present invention also relates to a method of making a rotary tool.
Background Information
A rotary tool upon which embodiments of the present invention improves is disclosed in U.S. Pat. No. 4,759,667, for example. Furthermore, a drilling tool having what is known as a four-surface ground section is also disclosed in U.S. Pat. No. 4,983,079.
The end surface of such drills having a multi-surface ground section typically has a first free surface segment adjoining the respective major cutting edge in the direction of rotation, said first free surface segment being oriented at a first clearance angle. Adjoining this is the second free surface segment which is oriented at a larger clearance angle. In the twist drill disclosed in U.S. Pat. No. 4,983,079, the first free surface segment runs along the entire major cutting edge. Two major cutting edges are thereby connected to one another via a chisel edge. To improve the cutting properties, according to U.S. Pat. No. 4,759,667 the first free surface segment is fashioned to be approximately triangular, with increasing width towards the circumference of the drilling tool. The first free surface segment begins not at the brad point, as is the case with conventional four-surface grinding, but rather radially distant from this point at the end of the chisel edge.
Owing to the edge fashioned by the grinding in the transition between the two free surface segments, mechanical stresses often occur at this edge. To avoid such mechanical stresses, a drill bit is fashioned—for example according to EP 1 230 058 B1—in which the free surface is fashioned as a surface without an edge, curving from the major cutting edge in the direction towards the flute.